In certain applications, such as radio applications, a conversion from a balanced signal to an unbalanced signal, or vice versa, is often required. Traditionally, the primary way to accomplish a balanced-unbalanced conversion is with a dedicated component. Having a separate component increases the requisite size and cost of such a device. Therefore it is advantageous if a conversion can be realized in another component.
An example of a component that is typically present in devices requiring balanced-unbalanced signal conversion is a filter. Several types of filter arrangements are known which can provide required signal conversion in addition to their standard required filter capabilities.
Bulk acoustic wave (BAW) filters, when arranged using vertical acoustic coupling (CRF), can potentially be used for balanced-unbalanced signal conversion. Balanced BAW filters are realized by vertically coupling two piezoelectric layers. However, due to the presence of two layers, these filters are extremely sensitive to the thickness variations between the piezoelectric layers. An additional disadvantage to using CRF BAW filters is that they are both difficult and expensive to manufacture.
Another type of filter that can be utilized is a surface acoustic wave (SAW) filter. Balun transformation, as well as impedance transformation, can be realized relatively easily in SAW components. However, compared to BAW, SAW components have much worse power handling capabilities. SAW filters have additional distinct disadvantages. SAW filters are often too large to be used in certain devices, specifically in small compact devices. Another disadvantage is that there are difficulties associated with patterning at high frequencies, e.g. above 2 GHz.
Perovskite metal-oxides, such as BaxSr1-xTiO3 (hereinafter called “BST”), show exceptional material properties including (i) switchable permanent polarization (ferroelectric phase), (ii) piezoelectricity (ferroelectric phase), (iii) field-dependent (tunable) polarization (paraelectric phase), (iv) low losses at high frequencies (paraelectric phase). Tunability and low losses, typically obtained in the paraelectric phase, make these materials of special interest for high-frequency applications. An example of a basic device realizable using these materials is a tunable capacitor (varactor).
For optimal tunability, parallel-plate device geometry is highly desirable: here the perovskite metal-oxide is sandwiched between two (top and bottom) metallic electrodes. Using planar device geometries based on metal electrodes deposited only on top of the perovskite metal-oxide, it is difficult to obtain sufficient tunability.
In the prior art parallel-plate capacitor fabrication process, the layers are deposited successively on a substrate, ie. the metallic bottom electrode is first deposited on the substrate, then the BST layer is deposited on the metallic bottom electrode and finally the metallic top electrode is deposited on the BST.
High processing temperatures are required for proper crystallization of perovskite oxides (e.g. for BST, Tmin˜650° C.). The high temperature and the presence of oxygen make the parallel-plate device processing problematic. The metallic bottom electrode is heavily attacked during BST growth. Typical problems encountered in the bottom electrode are (i) electrode destruction via oxidation, (ii) grain growth resulting in poor BST quality. On the other hand, the electrode material should be well conducting to prevent the critical losses at high frequencies. Of typical metals, Cu, Ag, Al, and Au are well conducting but either get easily oxidized and/or are unstable substrates for BST growth due to their low melting point. A typical state-of-the-art choice for the bottom electrode is Pt, which, however, (i) is a significantly worse conductor than the metals above, (ii) will easily exhibit grain growth if thick layers are employed to reduce conductor losses. Furthermore, Pt is permeable with respect to oxygen, and reactive with respect to other metals—both effects complicating the fabrication of multilayered electrode structures. The refractory metals (Mo, W) are more stable against grain growth and they are intermediate conductors but get easily oxidized. Protective layers (e.g. diffusion barriers) can in principle be employed to protect the bottom electrode from oxidation, but it is difficult to obtain a well-protecting layer with minimal parasitic effects (e.g. non-tunable capacitance).